You vaporize a liquid substance at 90.0 °C and 795.0 mmHg. The volume of 0.548 g of vapor is 237 mL. What is the molecular weight (in amu) of the substance?

Use PV = nRT and solve for n = number of mols.

Then mols = ? Don't forget to change T to kelvin and use P in atm.
Then mols = grams/molar mass. You know mols and grams, solve for molar mass.